Bug in computing widths of grid column headers?

We believe there may be a bug in how Ext computes the grid widths. Specifically, when we set “enableRowBody: true” in the ViewConfig, and then insert a child grid into the parent row’s body, we see Ext renders the child grid with the column headers not lining up with the columns in the rows. Drilling down in the output HTML, we discovered Ext sets the width of the child grid’s header to be 18 pixels wider than the width of child grid’s rows. As a result, the columns don’t line up with the headers. Manually editing the HTML to add those 18 pixels to the width fixes everything. Is there a workaround for this? Any idea why Ext would be inconsistent about the widths?
We first discovered this issue while using the RowExpander plugin, but to rule that out, we stripped down the code to remove the extension and produce this code here. Look at the columns on the right (“Child Col K”) and see how far off the alignment is.
Here is the code and the output HTML.

And here is the relevant section of the generated HTML. Note that the header has a width of 1251px, but the rows have a width of 1233px. That seems to be the problem. But where is the width coming from?